fastboot: Fix "fastboot gsi".

This command erroneously reports that no GSI is installed, because
/metadata is not mounted in recovery. To address this, temporarily mount
/metadata when the gsi command is invoked.

Bug: 122556707
Test: fastboot gsi disable
      fastboot gsi wipe

Change-Id: Ib21971b49b46fd580b902ff75f01cfb96192afc0